We are studying the chronic regulation of mitochondrial enzyme activity in muscle, with a view to understanding the weakness and decreased capacity for endurance training of skeletal muscle in old age. We are particularly concerned with mechanisms co-ordinating the expression of mitochondrial and nuclear genomes in the biogenesis of mitochondria. The approach this year has been two-fold. First, we have completed the studies of mitochondrial proliferation in response to differentiation of C2C12 (a mouse muscle cell line) and NIH 3T3 (a pre-adipocyte line) cells, which we introduced last year. In general, the same lack of co- ordination of the genomes appeared when messages (m-RNA) for nuclear-encoded and mitochondrial-encoded proteins were measured as was reported last year for enzyme activities. However, mt-DNA encoded messages, (COX I, II; ATP-ase 6) behaved as a group, as did nuclear-encoded messages (PDH E1alpha; CS). Insertion of newly-synthesized enzyme proteins into the mitochondrial inner membrane may be limited by available membrane area, based upon our quantitative morphometry of cristael membrane area per mitochondrion and per cell. In our second project, we have approached the question of why cytochrome c oxidase activities are reduced in the aging heart. Activity was found to be decreased by 25% in heart mitochondria from 24mo rat versus 6mo animals (3.3 and 4.8 mugatoms O2/min/mg). Rates of protein synthesis by isolated mitochondria were found to be decreased to a similar extent (27 and 42 pmol/mg/hr for old and young, respectively). Northern blots for COX II message in whole- heart extracts also reveal a decrement at 24mos, suggesting that transcription of mt- DNA is less active in aging and is limiting for mitochondrial biogenesis.